1. Field of the Invention
The present invention relates to a DC/DC converter and the like, and more particularly to a multi-output switching power supply.
2. Description of the Related Art
Conventionally, for example, in digital equipment such as a computer and a display device thereof, several power-supply voltages are used. In order to supply several power-supply voltages, a composite power supply may be made by combining a plurality of single output switching power supplies. However, for a relatively small output power supply, the overall size becomes large and the cost becomes high in spite of the small output capacity. Accordingly, in a power supply that is integrated into equipment requiring several power-supply voltages, a multi-output type switching power supply is used, which is configured to provide several output voltages with a single power conversion circuit.
FIG. 11 shows an example of a conventional circuit of a typical multi-output type switching power supply. In the figure, T represents a transformer having five windings L1, L2, L21, L22, and L23. L1 represents an input winding, L2 represents a feedback winding, and each of L21, L22, and L23 represents an output winding. A switching transistor Q1 is connected to one end of the winding L1, and an input voltage Vin is applied to the other end of the winding L1 from terminal Vin. A feedback circuit constituted of a capacitor C1 and a resistor R2 is provided between the feedback winding L2 and a base of the switching transistor Q1. A rectifying-smoothing circuit constituted of rectifying diodes D11 to D13 and smoothing capacitors C11 to C13 is connected to the output windings L21 to L23. As a result, different output voltages are generated at each of V1 to V3 output terminals.
With the miniaturization of electronic equipment in recent years, there is an increasingly strong demand for miniaturization of the power supply that is integrated into the electronic equipment. For miniaturization of a switching power supply, a transformer that is a main part thereof must be miniaturized. However, there is a problem in that many terminals cannot be provided therewith even though the main part of the transformer may be miniaturized to some extent. For example, in a miniaturized transformer with a low output, the number of terminals is limited to four.
However, in the case of the multi-output type switching power, as shown in FIG. 11, for example, in order to output three different voltages, it is required that the transformer is provided with seven terminals, and as a result, the transformer cannot be miniaturized, thus preventing miniaturization of the multi-output type switching power supply.
Accordingly, it is an object of the present invention to provide a switching power supply which is capable of generating many different output voltages and which can be easily miniaturized by using a transformer with a small numbers of terminals.
The above-mentioned object of the present invention can be achieved by a switching power supply including a transformer provided with at least two windings connected in series and no more than four terminals, a switching element for interrupting a current of an input winding of the transformer, a feedback circuit for providing a feedback signal from a feedback winding of the transformer to a control terminal of the switching element, a rectifying-smoothing circuit for rectifying and smoothing an induced voltage of an output winding of the transformer, and a plurality of rectifying smoothing circuits commonly connected to a predetermined terminal of said transformer so as to generate three or more different output voltages.
With this configuration, three or more different output voltages can be generated using a transformer with four or fewer terminals, thereby obtaining a compact switching power supply using a compact transformer.
The above-mentioned object of the present invention can be achieved by a switching power supply including a transformer provided with at least two windings connected in series and no more than four terminals, a switching element for interrupting a current of an input winding of the transformer, a feedback circuit for providing a feedback signal from a feedback winding of the transformer to a control terminal of the switching element, a rectifying-smoothing circuit for rectifying and smoothing an induced voltage of an output winding of the transformer, and rectifying-smoothing circuits connected to the feedback winding, which is connected to the feedback circuit, and to other windings, respectively, so as to generate three or more different output voltages. Thus, the feedback winding is also used for the output winding by connecting the rectifying-smoothing circuits to the feedback winding that is connected to the feedback circuit.
Usually, since the ratio of the number of turns of the feedback winding to the input winding can be made equal to or less than 1, a lower voltage can be obtained at an output side of a rectifying-smoothing circuit that is connected to the feedback winding than an output voltage that is generated by interruption of a current for the input winding and by an inductance of that input winding, thereby providing a switching power supply that generates a wide range of output voltages.
Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings.